The present invention generally relates to apparatus and methods for the bioprocessing of wastewater and, more specifically, to an improved bioprocessing system and method of processing wastewater that can be used in a micro-gravity environment.
A bioprocessor or bioreactor can be broadly defined as a container with a bio-substance therein. As a chemically active biological substance, it can be one of many nonliving substances and living microorganisms. For example, the bio-substance may be a chemical such as an enzyme or hormone or a living entity such as a bacterial or viral species. The container of the bioreactor has taken various forms, including items as simple as a vat to more complex items like porous elements and microcapsules.
Bioprocessors have frequently been used to grow useful cells or to clean contaminated effluent, such as water. For the treatment of contaminated water, the bioprocessor has been an alternative to the use of physical methods and chemical methods. The physical methods have included carbon adsorption onto activated charcoal units, air stripping and membrane separation. Some of the chemical methods have included precipitation and oxidation/reduction.
Biological methods can include aerobic or anaerobic processes. Aerobic processes have been more commonly employed due to their effectiveness in converting contaminants into less harmful substances. More specifically, biofilms have been widely used because an active biomass produced in the reactor allows large volumetric loadings and good effluent quality without the need for solids separation. The biofilm bioreactors have been generally categorized as continuously stirred tank reactors (CSTRs), fixed-bed and fluidized bed.
In the CSTR, the liquid (i.e., waste water) is completely mixed, such as by mechanical stirring, while an activated biomass grows and uniformly contacts the liquid without attachment to a media. In the fixed-bed reactor, the biofilm attaches to an immobile solid media while the liquid passes through the reactor. In the fluidized bed reactor, the liquid flows through the reactor at a sufficiently high rate to fluidize the solid media. Thereby, the solid media and attached biofilm are mixed throughout the reactor.
In both the fixed-bed and fluidized-bed bioreactors, the effluent can be recycled. A primary effect of recycling is to dilute the feed. Furthermore, evenly distributing the feed or nutrient throughout the bioreactor promotes more uniform growth of the biofilm throughout the reactor.
In fixed-bed bioreactors, the bioreactors can be classified as hollow fiber membrane or packed bed. In the former, a tubular or a hollow fiber membrane is used as the carrier or medium on which the biofilm can grow. In the latter, ceramic porous bodies in multi-layered plates, or hydrophobic polyurethane foams and pall rings (as in U.S. Pat. No. 5,217,616), have been used as the medium. In the hollow fiber membrane bioreactor, oxygen or air is transported through the lumens of the hollow fibers by a pressure gradient applied to the membrane interfaces. The biofilm is typically grown in the fluid space between the outer shells of the hollow fiber membrane and the shell wall of the bioreactor.
The oxygen required for biofilm growth can be supplied by several commonly used methods such as aeration through bubble diffusers and permeation through membranes. Oxygen permeates or transports through a non-porous polymer membrane when a pressure gradient is applied between the two interfaces of the membrane. The permeation process occurs by a solution-diffusion mechanism that is commonly controlled by the molecular diffusion of oxygen in the polymer matrix of the membrane. At the same time, solution equilibrium is established between the oxygen molecules in a gas phase in contact with the membrane interfaces and the molecules dissolved in the polymer at those interfaces. If one of the membrane interfaces is in contact with a liquid phase, then oxygen transfer from the gas phase to the liquid phase through the membrane also occurs by the solution-diffusion mechanism. A bubbleless oxygen transfer to the liquid phase can then be achieved by controlling the gas phase oxygen pressure. In a micro-gravity environment, oxygenation of the liquid phase through a conventional bubble diffuser method is no longer possible due to the difficulties associated with phase separation. Utilization of a membrane to oxygenate a liquid in a micro-gravity environment eliminates the two-phase fluid flow problem.
Irrespective of the type of bioreactor, a single bioreactor is of course limited in utility by its size. Thus, attempts have been made to combine multiple bioreactors to scale up the overall system utility. An early example of using multiple bioreactors is shown in U.S. Pat. No. 3,821,087. Therein, a bioreactor is of the hollow fiber type through which a liquid nutrient medium can flow. A plurality of bioreactors can be utilized in a parallel or serial arrangement. A similar parallel or serial arrangement is described in U.S. Pat. No. 4,266,026. However, the serial or parallel arrangements in those patents tend to increase the size of the overall system and do not provide for both aerobic and anaerobic processing.
A wastewater treatment system in U.S. Pat. No. 4,279,753 includes three aerobic-anaerobic bioreactors operating in series. In each aerobic-anaerobic bioreactor, wastewater first flows through an aerobic portion and then downwards to anaerobic portion wherein the flow is received for upward movement. The series of bioreactors is intended to provide incremental consumption of organic nutrients, nitrification and denitrification. However, the need for a plurality of the same type of bioreactors arranged in series increases the overall size of the system. And the need for upward and downward flows of the wastewater makes it unsuitable for a micro-gravity environment.
Another wastewater treatment system is shown in U.S. Pat. No. 5,578,214. A first bioreactor includes an anaerobic lower portion where the feed is introduced. From the anaerobic lower portion, the feed moves up and into an aerobic-anaerobic upper portion. A second bioreactor again provides aerobic processing. But since there is no apparent recycling of the feed, system efficiency is compromised, no self controlling of pH is employed, and in the anaerobic zone generation of gaseous products such as methane and hydrogen sulfide will occur.
U.S. Pat. No. 5,702,604 discloses three bioreactors operating in series with one another. The first bioreactor includes an anaerobic portion for initial treatment and an aerobic portion for subsequent treatment. The second bioreactor includes an aerobic portion that receives wastewater from the aerobic portion of the first bioreactor. From the second bioreactor, the wastewater can move into a denitrification tank or be recirculated back to the first bioreactor. The third bioreactor receives wastewater from the denitrification tank and includes a hydroponic portion within which plants can be cultivated. Disadvantages, however, include the need for multiple types of bioreactors and tanks that increase the complexity of the system. Also complicating the system is a combined anaerobic-aerobic bioreactor that requires upward and downward flows of the wastewater, a partition to separate aerobic and anaerobic processing, and a filter that separates the anaerobic-aerobic processing from exhaust gas processing.
In another wastewater treatment system utilizing a series of bioreactors, U.S. Pat. No. 5,582,732 provides an anaerobic bioreactor, an anoxic bioreactor, an aerobic bioreactor, and then a nitrification bioreactor in series. In the anaerobic bioreactor, there are apparently no nitrites or nitrates undergoing conversion (i.e., denitrification). Instead, it is in the separate anoxic bioreactor that nitrites and nitrates are converted into nitrogen gas and nitrous oxide. The aerobic bioreactor appears to be essentially involved in total organic carbon (TOC) degradation, but with little or no nitrification. Consequently, the separate nitrification bioreactor is provided wherein nitrites and nitrates are produced and can be routed back to the anoxic bioreactor. Little, if any, TOC degradation apparently occurs in the nitrification bioreactor. Disadvantages to the system include the need for multiple types of bioreactors carrying out separate functions or steps, as opposed to combined functions or steps. Aeration occurs outside the aerobic bioreactor by a venturi bubble system that complicates the aeration process and makes the system micro-gravity incompatible. Also, only about 30% of the system is plug flow while about 70% is operated as a CSTR which, in turn, increases the total system size.
Supra et al., "Advancements in Regenerative Life Support Waste Water Bioprocessing Technology," SAE Technical Paper Series, 26.sup.th International Conference on Environmental Systems (1996) disclose a system for processing gray water by the use of an immobilized cell bioreactor that provides a majority of TOC degradation and nitrification. An effluent from the bioreactor is recycled to a roughing filter that is primarily designed to remove particulates and achieves about 30% TOC degradation. However, since the roughing filter only achieves a small amount of TOC degradation, the overall efficiency of the system may be limited.
As another example of recycling, U.S. Pat. No. 4,804,628 utilizes a hollow fiber cartridge containing a plurality of capillaries as a bioreactor, which is similar to U.S. Pat. Nos. 3,821,087 and 4,266,026. Upon a feed being delivered through the bioreactor, the feed is re-circulated back to a delivery system for recycling into the bioreactor. In part, the re-circulation is intended to assist in minimizing the formation of nutrient gradients, minimizing the formation of microenvironments around metabolizing cells, and minimizing pressure drops along the length of the capillaries. Yet, disadvantages to the system include the fact that both aerobic and anaerobic processes are not simultaneously provided. U.S. Pat. No. 5,656,421 also shows the use of parallel operating bioreactors wherein a flow is re-circulated through the bioreactors.
As can be seen, there is a need for improved bioprocessing equipment and methods. In particular, there is a need for apparatus and methods that reduce the overall system size by, for example, combining steps or functions in a single bioreactor or subsystem, while maintaining or increasing system efficiency. A further need is for a system that provides both aerobic and anaerobic processing, while maintaining simplicity in system design. Also needed is a bioprocessor system which improves efficiency by utilizing the system's own by-products to take the place of an oxidizer which otherwise needs to be added to the system. Another need is for improved pH control for system optimization. Yet another need is for a system that is not dependent upon gravity for oxygenation.